Scientific Forecaster Discussion
fxus62 kchs 170824
Area forecast discussion
National Weather Service Charleston SC
424 am EDT Tue Sep 17 2019
Hurricane Humberto will continue to move away from the coast
through mid week. A cold front will advance through the area
Wednesday, followed by high pressure into early next week.
Near term /through tonight/...
today: aloft, the primary feature of interest is the high
amplitude ridge that extends from the lower Mississippi Valley
northward into central Canada. At the surface, Humberto will
begin to accelerate to the east while a cold front approaches
the the forecast area from the north late in the day. The
forecast is dry through the day ahead of the front. The main
story will be the continued well above normal temperatures, and
at least a low end chance of record highs. The forecast depicts
highs ranging from the low to mid 90s across southeast South
Carolina and from the mid to even upper 90s for southeast
Georgia. Such values would be on the order of 7-10 degrees above
normal. Based on daily high temperatures records (see climate
section below), Savannah will be very close while Charleston is
expected to fall a few degrees short. Nonetheless, a very hot
day for mid September.
Tonight: the cold front will steadily drop southward through
the forecast area, likely reaching the Altamaha by sunrise
Wednesday. The biggest forecast challenge will be if any showers
can develop along and immediately ahead of the front, and what
will the coverage look like through the night. Given that the
front is moving in from the north with a relatively limited
moisture field at a climatologically unfavorable time for
convection, conventional Wisdom would favor minimal coverage.
Fortunately, most model guidance agrees with this with a limited
precipitation response. Therefore, the forecast features rain
chances around 20 percent in most areas late tonight with a
small tier of 30 percents over an area from Beaufort County
south including the adjacent coastal waters. There isn't much
support for thunder, so we have handled it with just showers.
Overnight lows should range from around 70 across Berkeley
County to the low 70s southward to the Altamaha river.
Short term /Wednesday through Friday/...
wednesday: the timing of a backdoor cold front through the
forecast area has generally remained consistent on recent model
runs and we maintained good persistence with the mid week
forecast. The temporary increase in moisture will likely result
in scattered showers across parts of the area around mid
morning, then chances will shift to the south of I-16 by the
afternoon as dew points fall from the north in the wake of the
front. Our only changes were to bump up pops to modest chance
ranges closer to the Altamaha river area. Temps will not be
nearly as warm with highs ranging from low to mid 80s north of
I-16 with some close to 90 degree readings possible south of a
line from Metter to Ludowici Georgia.
Thursday: cooler high pressure to the north at the surface will
wedge to the Lee of the mountains, providing temps that should
run a bit below climo with the upper ridge displaced to the
west. It should also be a bit breezy along the immediate coast
with a tight pressure gradient across the area. Sunshine should
prevail as temps reach the upper 70s far north to the lower 80s
much of southeast Georgia.
Friday: deeper ridging will develop as mid-level heights build
over the region. In the lower levels, high pressure will reside
across the mid Atlantic states. We did introduce a slight chance
for a few light showers coming in off the ocean into coastal
Georgia. Since 850-500 mb condensation pressure deficits are still
pretty high, we don't expect much measurable pcpn if at all in
this area. Temps will remain constant for the most part with
highs reaching 80 to 85 degrees from north to south after
morning lows that may dip into the mid-upper 50s in cooler spots
along our northwest tier of zones.
Long term /Friday night through Monday/...
deep layered ridging will persist over the weekend and then
tend to break down during the first part of next week. Models
continue to show dry weather prevailing through the entire
period. High temps are expected to warm slightly above climo
during the period, however overnight temps should continue to
fall into the 60s each night away from the ocean as the
atmosphere remains quite dry.
Aviation /08z Tuesday through Saturday/...
VFR conditions will prevail at kchs and ksav through 06z
Extended aviation outlook: there are chances for at least
patchy MVFR cigs through the southern portion of the coastal
corridor Wednesday morning. Otherwise, VFR conditions will
today through tonight: winds will continue to improve across
the local waters, with north to northeast flow at 15 knots or
less. Overnight, winds will start to pick back up as a cold
front pushes southward. By sunrise Wednesday expect to see 15-20
knots for the South Carolina waters, with 10-15 further south.
Seas during the day will run 4-6 feet in the Charleston County
waters and 4-7 feet in the outer Georgia waters. Elsewhere
expect 3-5 feet. Overnight seas will begin to tick upward again
late as winds pick up behind the front. The Small Craft Advisory
remains in effect for the Charleston County waters and the
outer Georgia waters. A new Small Craft Advisory for the
southern South Carolina waters has been hoisted starting after
Northeast flow will dominate throughout the week in the wake of
the backdoor cold front on Wednesday. The initial surge of
winds will develop southward over the waters and Small Craft Advisory conditions
will likely reach the Georgia near shore waters by afternoon at the
latest as seas build to 4-6 ft. Small Craft Advisory conditions are expected all
waters into late week as winds and waves remain elevated with
gradients tightened by high pressure to the north. Winds are
expected to lighten by the weekend with seas coming down a bit
more slowly than wind speeds.
Rip currents: moderate risk at all beaches today due mainly to
earlier rip current reports and continued elevated/long period
swell emanating from Hurricane Humberto. The rip current risk
will then remain elevated through the end of the week as
northeast winds and the longshore currents strengthen behind the
mid-week backdoor cold front.
low chances for minor coastal flooding around the times of high
tide through late week, especially along the South Carolina
daily record high temperatures for Tuesday, 9/17.
Charleston International Airport (kchs)
Savannah/Hilton Head international Airport (ksav)
97...1972, 1933, 1896
Downtown Charleston (kcxm)
Marine...Small Craft Advisory from 2 am Wednesday to 8 PM EDT Friday
Small Craft Advisory until 4 am EDT Sunday for amz374.
Small Craft Advisory until 6 PM EDT Friday for amz350.